EP0893701A1 - Spread spectrum MRI - Google Patents
Spread spectrum MRI Download PDFInfo
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- EP0893701A1 EP0893701A1 EP98305679A EP98305679A EP0893701A1 EP 0893701 A1 EP0893701 A1 EP 0893701A1 EP 98305679 A EP98305679 A EP 98305679A EP 98305679 A EP98305679 A EP 98305679A EP 0893701 A1 EP0893701 A1 EP 0893701A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/483—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy
- G01R33/4833—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/3806—Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/46—NMR spectroscopy
- G01R33/4625—Processing of acquired signals, e.g. elimination of phase errors, baseline fitting, chemometric analysis
Definitions
- the present invention pertains generally to methods for acquiring data that is useful for magnetic resonance imaging (MRI). More particularly, the present invention pertains to methods and techniques for acquiring image data for magnetic resonance imaging when the magnetic field is characterized by a z-gradient.
- the present invention is particularly, but not exclusively, useful for employing spread spectrum techniques in magnetic resonance imaging to suppress exogenous noise, to make MRI signals immune to static field perturbations and to overcome perceived inefficiencies of nonhomogeneous magnetic fields.
- Magnetic resonance imaging is a well known and widely used method for obtaining medical images for both diagnostic and research purposes.
- a volume of tissue is first placed in a static magnetic field.
- the tissue is then irradiated with radio frequency energy to tilt the nuclear magnetic moments within the tissue.
- Spin echo signals which are characteristic of the irradiated tissue, are then recorded from the tilted nuclear magnetic moments.
- the signal contributions of individual volume elements (voxels) in the tissue are distinguished from each other. These voxels are ultimately displayed on a computer monitor or film for use by the physician or researcher.
- the magnetic field at that particular point can be characterized by the respective x, y and z components of the field and by spatial derivatives of the field strength.
- the x, y and z components of the field magnetic vector, B 0 are designated B x , B y and B z .
- the magnetic field can then be further characterized by the gradients which are the rate of change (first derivatives) of the field strength in the x, y and z directions.
- the field gradients are designated G x , G y and G z . It is to be appreciated that higher order derivatives may, and most likely are, present. For purposes of discussing the present invention, however, only the field gradients G x , G y and G z need be considered.
- a homogeneous magnetic field exists in a small neighborhood of a point where all of the field gradients, i.e. G x , G y and G z , are zero.
- the effect of this range of Larmor frequencies is a spatial pattern of phase accumulation in the magnetic moments across the tissue of interest.
- the key point is that the range of Larmor frequencies associated with a gradient field (G x or G y ) spatially distinguish one voxel from another in the respective x and y directions.
- the present invention contemplates an MRI operation with a z-gradient (G z ), several consequences which involve data acquisition and the suppression of exogenous noise are pertinent.
- G z z-gradient
- the data acquisition techniques in the presence of a z-gradient are quite different from those used for conventional MRI in a homogeneous magnetic field. This data acquisition aspect has been fully considered and disclosed in U.S. Patent No. 5,304,930, which has been cited above and which is incorporated herein by reference.
- the suppression of exogenous noise is accomplished by imposing a z-gradient, G z , which is greater than either the x or y encoding gradients (G x and G y ).
- G z z-gradient
- An additional benefit from this relationship between the gradients is the fact that the system is less sensitive to static field perturbations.
- Spread spectrum techniques are widely used in the communications industry to avoid the corruption of transmitted signals by interfering noise sources.
- the method is particularly effective in the presence of a discrete set of noise sources that occupy narrow frequency bands.
- Conventional radio or television signals fall into this category.
- the basic idea in spread spectrum techniques is to send and receive signals that occupy a range of frequencies that is significantly wider than that of the individual interfering noise sources. In this manner, the effects of the individual noise sources are minimized.
- an object of the present invention to provide methods for acquiring data from voxels in a contour of tissue which is accomplished using a z-gradient, G z . It is another object of the present invention to provide methods for acquiring data from voxels in a contour of tissue which uses an extensive z-gradient, G z , to suppress exogenous noise making the data less sensitive to static field perturbations. Still another object of the present invention is to provide methods for MRI which are relatively easy to accomplish and comparatively cost effective.
- a method in accordance with the present invention involves positioning a tissue sample to be imaged in a magnetic field.
- a nonhomogeneous magnetic field will have a permanent z-gradient (G z ) which may be intentionally imposed, but is most typically an inherent characteristic of the magnet system that generates the magnetic field.
- a slice or contour of the tissue sample is excited by the application of RF energy.
- the transmission of this RF energy corresponds to the range of Larmor frequencies in the contour, which will be the same for all voxels in the contour.
- the tissue sample is then subsequently impressed with an x-gradient (G x ) and a y-gradient (G y ).
- the effect of impressing the x and y gradients (G x , G y ) on the tissue sample is to encode a portion of the tissue as a contour of voxels.
- each of the voxels will have its own slightly different field strengths in the x and y directions. It is this spatial information which distinguishes one voxel from another voxel in the contour.
- all of the voxels in the contour are subjected to the same z-gradient.
- the effect of the z-gradient is to spread the spectrum of each of the encoded voxels.
- the z-gradient (G z ) is typically orders of magnitude (i.e. ten to one hundred times) larger than either the x-gradient (G x ) or the y-gradient (G y ).
- tilted nuclei generate spin echo signals which contain informational data that can be received and processed to image the tissue.
- the x, y and z gradients are appropriately activated, and the nuclei in the tilted and encoded voxels are refocused to form spin echoes which are received for data acquisition purposes.
- the data acquisition process envisioned by the present invention involves a series of refocusing pulses which are applied at a rate that is proportional to the z-gradient.
- the z- gradient does not impart encoding patterns. Nevertheless, the z-gradient causes all of the voxels' spins to oscillate at a same range of Larmor frequencies. Further, the extensive z-gradient allows use of spread spectrum techniques for the suppression of exogenous noise and desensitization to static field perturbations.
- a magnet system in accordance with the present invention is shown and generally designated 10.
- the magnet system includes a North pole face 12 and a South pole face 14 which are both mounted on a base 16.
- North pole face 12 and the South pole face 14 generate a magnetic field which is represented by the magnetic field lines 18.
- the magnetic field is nonhomogeneous and, with reference to the orthogonal x-y-z coordinate system, the magnetic field has an inherent permanent z-gradient, G z .
- G z will be above approximately 0.2 gauss per millimeter and, most likely, will be somewhere around 3 gauss per millimeter.
- Figure 1 also shows that the system 10 includes a transmitting antenna system 20 and a receiving antenna system 22 which are both mounted on the base 16.
- a transmitting antenna system 20 and a receiving antenna system 22 which are both mounted on the base 16.
- many types of antenna systems 20, 22 can be used either separately or in combination in any manner well known in the art.
- the antenna systems 20, 22 are connected to a computer 24 which will control the transmission and reception of signals from the antenna systems 20, 22.
- the magnet system 10 is designed with unshielded pole faces 12, 14 which are small enough to make the system 10 portable. Consequently, in addition to the permanent z-gradient, the magnetic field 18 will also be subjected to unpredictable exogenous noise and static field perturbation which are represented by the arrows 26 in Figure 1.
- FIG 1 also shows a tissue sample 28 (e.g. a hand) that has been positioned in the magnetic field of magnet system 10. It is to be appreciated that the tissue sample 28 can be oriented as desired. In all cases, however, the portion of tissue which will be imaged lies in an x-y plane, a slice or contour, which is perpendicular to the z axis and which is, thus, also perpendicular to the z-gradient (G z ).
- G z z-gradient
- Figure 2 shows a representative contour 30 from the tissue sample 28 which is to be imaged.
- the contour 30 includes a plurality of voxels 32 of which the voxel 32a is representative.
- the contour 30 and the voxels 32 therein are bonded by an upper surface 34 and a lower surface 36.
- the contour 30 has a thickness t.
- the surfaces 34, 36 are each of constant field magnitude, B OU and B OL respectively, and, consequently, the surfaces have different Larmor frequencies, f OU and f OL respectively.
- the range of Larmor frequencies (f OU - f OL ) across the thickness t of the contour 30 is due to G z .
- G z will be greater than either G x or G y , and, in most cases, much greater.
- G z is permanent.
- G z will be imposed on the tissue sample 28 as soon as the tissue sample 28 is positioned in the magnetic field.
- G z is the same for all of the voxels 32 in contour 30 and, thus, the range of Larmor frequencies will be the same for all voxels 32. Therefore, as used for the present invention, G z is, an intravoxel gradient.
- G x and G y are, in contrast, intervoxel gradients which serve to encode and thereby spatially differentiate the voxels 32 from each other.
- a method for operation in accordance with the present invention involves positioning a tissue sample 28 in the magnetic field of a magnet system 10.
- This magnetic field is characterized by a z-gradient, G z .
- G z is preferably substantially constant and sufficiently large to suppress exogenous noise with spread spectrum techniques.
- Figure 3 shows how the intravoxel range of Larmor frequencies, which result from G z , are spread in spectrum.
- the spectral density 38 resulting from the intravoxel gradient (G z ) is rather extensive.
- spectral densities of interfering exogenous noise sources and static field perturbations shown by the lines 40 a-c, are small relative to the spread in signal spectrum.
- the antenna system 20 is activated to tilt nuclei in the tissue sample 28.
- the tissue sample 28 is encoded with x and y gradients.
- the result is a contour 30 of encoded voxels 32.
- the contour 30 is then irradiated with refocusing pulses which cause the nuclei to generate receivable spin echo signals. All of the actions; tilting, encoding, and refocusing are controlled by the computer 24.
- the resultant spin echo signals are then received by the antenna system and passed to computer 24 where they are processed as desired.
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Abstract
A method for acquiring data from voxels in a contour of tissue includes
positioning the tissue in a magnetic field. For the present method, a z-gradient
is imposed on the contour tissue to spread the spectrum of all of the
voxels over a same range of Larmor frequencies. Additionally, voxels in the
contour tissue are selectively encoded with different x-gradients and y-gradients
to distinguish the various voxels from each other. In the presence
of the z-gradient, nuclei of the encoded voxel are tilted and then refocused at
a rate proportional to the z-gradient. Due to this refocusing, spin echo
signals are generated which are useful for acquiring data from the tissue.
The intravoxel z-gradient used for the present invention is the same for all
voxels and is greater than either the x-gradient or the y-gradient which are
used for encoding. The z-gradient may be substantially constant.
Importantly, the z-gradient is sufficiently extensive to spread the spectrum of
the spin echo signals for suppression of exogenous noise and make the
signals immune to static field perturbations.
Description
The present invention pertains generally to methods for acquiring data
that is useful for magnetic resonance imaging (MRI). More particularly, the
present invention pertains to methods and techniques for acquiring image
data for magnetic resonance imaging when the magnetic field is
characterized by a z-gradient. The present invention is particularly, but not
exclusively, useful for employing spread spectrum techniques in magnetic
resonance imaging to suppress exogenous noise, to make MRI signals
immune to static field perturbations and to overcome perceived inefficiencies
of nonhomogeneous magnetic fields.
Magnetic resonance imaging is a well known and widely used method
for obtaining medical images for both diagnostic and research purposes. In
order to conduct a typical MRI procedure a volume of tissue is first placed in
a static magnetic field. The tissue is then irradiated with radio frequency
energy to tilt the nuclear magnetic moments within the tissue. Spin echo
signals, which are characteristic of the irradiated tissue, are then recorded
from the tilted nuclear magnetic moments. By using imaging techniques well
known in the art, the signal contributions of individual volume elements
(voxels) in the tissue are distinguished from each other. These voxels are
ultimately displayed on a computer monitor or film for use by the physician or
researcher.
For any system which relies on nuclear magnetic resonance
techniques for acquiring data, one very important design consideration is the
system's static magnetic field. For purposes of discussing the static magnetic
field, consider an orthogonal x-y-z coordinate system. With the origin of this
coordinate system at any point in the magnetic field, the magnetic field at that
particular point can be characterized by the respective x, y and z components
of the field and by spatial derivatives of the field strength. Specifically, the x,
y and z components of the field magnetic vector, B0, are designated Bx, By
and Bz. The magnetic field can then be further characterized by the gradients
which are the rate of change (first derivatives) of the field strength in the x, y
and z directions. The field gradients are designated Gx, Gy and Gz. It is to be
appreciated that higher order derivatives may, and most likely are, present.
For purposes of discussing the present invention, however, only the field
gradients Gx, Gy and Gz need be considered.
In a very general sense, a homogeneous magnetic field exists in a
small neighborhood of a point where all of the field gradients, i.e. Gx, Gy and
Gz, are zero.
To by-pass the difficulties encountered with the manufacture of
homogeneous MRI systems, recent efforts have been made to effectively use
the more commonplace and more cost effectively established
nonhomogeneous magnetic field. For example, U.S. Patent No. 5,304,930
which is assigned to the assignee of the present invention, and which issued
to Crowley et al. for an invention entitled "Remotely Positioned MRI System"
(hereinafter the '930 patent) discloses a device and method for magnetic
resonance imaging with a nonhomogeneous magnetic field. As clearly
disclosed in the '930 patent, a nonhomogeneous field is a field that has a
non-zero gradient Gz. Regardless whether the magnetic field is
homogeneous or nonhomogeneous, in order to perform an MRI procedure it
is necessary to distinguish various voxels within the tissue to be imaged. To
do this, the tissue is typically encoded with spatial patterns.
One widely recognized encoding procedure for imparting spatial
patterns in the tissue volume to be imaged involves the application of
gradient magnetic fields. These so-called gradient magnetic fields consist of
an additional range of field values, denoted by Δ Bo, that are superimposed
on the static field. At this point it should be noted that the x and y spatial
variations of Δ Bo are determined by the respective x and y gradients, Gx and
Gy, of the superimposed field values. Through the Larmor constant, a range
of Larmor frequencies determined by the expression
Δ f = λ Δ Bo
exists during the application of the gradient, either Gx or Gy or both. The
effect of this range of Larmor frequencies is a spatial pattern of phase
accumulation in the magnetic moments across the tissue of interest. For the
purposes of the present invention, the key point is that the range of Larmor
frequencies associated with a gradient field (Gx or Gy) spatially distinguish
one voxel from another in the respective x and y directions. With a suitable
number of gradient encodings, which are each followed by a measurement of
spin echo signals, data is obtained that can be reconstructed into an image of
the array of voxels.
Due to the fact the present invention contemplates an MRI operation
with a z-gradient (Gz), several consequences which involve data acquisition
and the suppression of exogenous noise are pertinent. First, the data
acquisition techniques in the presence of a z-gradient are quite different from
those used for conventional MRI in a homogeneous magnetic field. This data
acquisition aspect has been fully considered and disclosed in U.S. Patent No.
5,304,930, which has been cited above and which is incorporated herein by
reference. Second, the suppression of exogenous noise is accomplished by
imposing a z-gradient, Gz, which is greater than either the x or y encoding
gradients (Gx and Gy). An additional benefit from this relationship between
the gradients is the fact that the system is less sensitive to static field
perturbations.
Spread spectrum techniques are widely used in the communications
industry to avoid the corruption of transmitted signals by interfering noise
sources. The method is particularly effective in the presence of a discrete set
of noise sources that occupy narrow frequency bands. Conventional radio or
television signals fall into this category.
The basic idea in spread spectrum techniques is to send and receive
signals that occupy a range of frequencies that is significantly wider than that
of the individual interfering noise sources. In this manner, the effects of the
individual noise sources are minimized.
The use of such techniques is not conventionally taught in the art
since, as mentioned above, most magnetic resonance equipment can be
shielded from the effect of external noise by enclosing the system in an r.f.
shielded room (Faraday cage). However, the present invention recognizes
there are benefits to systems that are not enclosed in shielded rooms,
especially for cost of operation and smaller portable systems.
In light of the above, it is an object of the present invention to provide
methods for acquiring data from voxels in a contour of tissue which is
accomplished using a z-gradient, Gz. It is another object of the present
invention to provide methods for acquiring data from voxels in a contour of
tissue which uses an extensive z-gradient, Gz, to suppress exogenous noise
making the data less sensitive to static field perturbations. Still another
object of the present invention is to provide methods for MRI which are
relatively easy to accomplish and comparatively cost effective.
A method in accordance with the present invention involves positioning
a tissue sample to be imaged in a magnetic field. Specifically, a
nonhomogeneous magnetic field will have a permanent z-gradient (Gz) which
may be intentionally imposed, but is most typically an inherent characteristic
of the magnet system that generates the magnetic field.
Once the tissue sample has been positioned in the magnetic field and
the z-gradient, Gz, is imposed, a slice or contour of the tissue sample is
excited by the application of RF energy. The transmission of this RF energy
corresponds to the range of Larmor frequencies in the contour, which will be
the same for all voxels in the contour. The tissue sample is then
subsequently impressed with an x-gradient (Gx) and a y-gradient (Gy). The
effect of impressing the x and y gradients (Gx, Gy) on the tissue sample is to
encode a portion of the tissue as a contour of voxels. Due to this encode,
each of the voxels will have its own slightly different field strengths in the x
and y directions. It is this spatial information which distinguishes one voxel
from another voxel in the contour. On the other hand, as indicated above, all
of the voxels in the contour are subjected to the same z-gradient. Thus,
rather than distinguishing the voxels from each other, the effect of the z-gradient
is to spread the spectrum of each of the encoded voxels.
Importantly, the z-gradient (Gz) is typically orders of magnitude (i.e. ten to one
hundred times) larger than either the x-gradient (Gx) or the y-gradient (Gy).
As is well known in the art, tilted nuclei generate spin echo signals
which contain informational data that can be received and processed to
image the tissue. For the present invention, the x, y and z gradients are
appropriately activated, and the nuclei in the tilted and encoded voxels are
refocused to form spin echoes which are received for data acquisition
purposes. Specifically, as disclosed in great detail in the '930 patent, the
data acquisition process envisioned by the present invention involves a
series of refocusing pulses which are applied at a rate that is proportional to
the z-gradient.
Several aspects of the present invention are particularly noteworthy.
Firstly, the z- gradient does not impart encoding patterns. Nevertheless, the
z-gradient causes all of the voxels' spins to oscillate at a same range of
Larmor frequencies. Further, the extensive z-gradient allows use of spread
spectrum techniques for the suppression of exogenous noise and
desensitization to static field perturbations.
The novel features of this invention, as well as the invention itself, both
as to its structure and its operation, will be best understood from the
accompanying drawings, taken in conjunction with the accompanying
description, in which similar reference characters refer to similar parts, and in
which:
Referring initially to Figure 1, a magnet system in accordance with the
present invention is shown and generally designated 10. As shown, the
magnet system includes a North pole face 12 and a South pole face 14 which
are both mounted on a base 16. As mounted on the base 16, North pole face
12 and the South pole face 14 generate a magnetic field which is represented
by the magnetic field lines 18. In the preferred embodiment, the magnetic
field is nonhomogeneous and, with reference to the orthogonal x-y-z
coordinate system, the magnetic field has an inherent permanent z-gradient,
Gz. For purposes of the present invention, Gz will be above approximately 0.2
gauss per millimeter and, most likely, will be somewhere around 3 gauss per
millimeter.
Figure 1 also shows that the system 10 includes a transmitting
antenna system 20 and a receiving antenna system 22 which are both
mounted on the base 16. As will be appreciated by the skilled artisan, for the
purposes of the present invention many types of antenna systems 20, 22 can
be used either separately or in combination in any manner well known in the
art. In any case, the antenna systems 20, 22 are connected to a computer 24
which will control the transmission and reception of signals from the antenna
systems 20, 22.
As intended for the present invention, the magnet system 10 is
designed with unshielded pole faces 12, 14 which are small enough to make
the system 10 portable. Consequently, in addition to the permanent z-gradient,
the magnetic field 18 will also be subjected to unpredictable
exogenous noise and static field perturbation which are represented by the
arrows 26 in Figure 1.
Figure 1 also shows a tissue sample 28 (e.g. a hand) that has been
positioned in the magnetic field of magnet system 10. It is to be appreciated
that the tissue sample 28 can be oriented as desired. In all cases, however,
the portion of tissue which will be imaged lies in an x-y plane, a slice or
contour, which is perpendicular to the z axis and which is, thus, also
perpendicular to the z-gradient (Gz).
Figure 2 shows a representative contour 30 from the tissue sample 28
which is to be imaged. As shown, for a point 31 in the x-y-z coordinate
system, the contour 30 includes a plurality of voxels 32 of which the voxel
32a is representative. As also shown in Figure 2, the contour 30 and the
voxels 32 therein are bonded by an upper surface 34 and a lower surface 36.
Accordingly, the contour 30 has a thickness t. Importantly, the surfaces 34,
36 are each of constant field magnitude, BOU and BOL respectively, and,
consequently, the surfaces have different Larmor frequencies, fOU and fOL
respectively. As indicated by earlier disclosure, the range of Larmor
frequencies (fOU- fOL) across the thickness t of the contour 30 is due to Gz.
As stated above, Gz will be greater than either Gx or Gy, and, in most cases,
much greater. Also, as defined herein, Gz is permanent. Thus, Gz will be
imposed on the tissue sample 28 as soon as the tissue sample 28 is
positioned in the magnetic field. Further, Gz, is the same for all of the voxels
32 in contour 30 and, thus, the range of Larmor frequencies will be the same
for all voxels 32. Therefore, as used for the present invention, Gz is, an
intravoxel gradient. Gx and Gy are, in contrast, intervoxel gradients which
serve to encode and thereby spatially differentiate the voxels 32 from each
other.
A method for operation in accordance with the present invention
involves positioning a tissue sample 28 in the magnetic field of a magnet
system 10. This magnetic field is characterized by a z-gradient, Gz. Gz is
preferably substantially constant and sufficiently large to suppress exogenous
noise with spread spectrum techniques. Specifically, Figure 3 shows how the
intravoxel range of Larmor frequencies, which result from Gz, are spread in
spectrum. As indicated in Figure 3, the spectral density 38 resulting from the
intravoxel gradient (Gz), is rather extensive. In contrast, spectral densities of
interfering exogenous noise sources and static field perturbations, shown by
the lines 40 a-c, are small relative to the spread in signal spectrum.
Once the tissue sample 28 has been properly positioned, the antenna
system 20 is activated to tilt nuclei in the tissue sample 28. After the tilting of
the nuclei, or before if desired, the tissue sample 28 is encoded with x and y
gradients. The result is a contour 30 of encoded voxels 32. The contour 30
is then irradiated with refocusing pulses which cause the nuclei to generate
receivable spin echo signals. All of the actions; tilting, encoding, and
refocusing are controlled by the computer 24. The resultant spin echo
signals are then received by the antenna system and passed to computer 24
where they are processed as desired.
It is to be appreciated that the techniques disclosed herein are given in
the context of a nonhomogeneous magnetic field. These techniques,
however, are also applicable to homogeneous fields whenever a Gz is
superposed on the homogeneous field.
While the particular spread spectrum MRI, as herein shown and
disclosed in detail, is fully capable of obtaining the objects and providing the
advantages herein before stated, it is to be understood that it is merely
illustrative of the presently preferred embodiments of the invention and that
no limitations are intended to the details of construction or design herein
shown other than as described in the appended claims.
Claims (23)
- A method for acquiring data from voxels in a contour of tissue which comprises the steps of:positioning said tissue in a magnetic field;impressing an encode gradient on said tissue to encode said voxels;imposing a z-gradient on said encoded voxels to spread the spectrum of all said voxels over a same range of Larmor frequencies, said z-gradient being greater than said x-y encode gradients; andacquiring data from said voxels over said same range of Larmor frequencies.
- A method as recited in claim 1 further comprising the step of filtering said range of Larmor frequencies to attenuate exogenous noise.
- A method as recited in claim 1 wherein said magnetic field is homogeneous.
- A method as recited in claim 1 wherein said z-gradient is inherent and maintains a value greater than zero during said acquiring step.
- A method as recited in claim 4 wherein said z-gradient is substantially constant during said acquiring step.
- A method as recited in claim 1 wherein said z-gradient is greater than 0.2 gauss per millimeter.
- A method as recited in claim 1 wherein said contour has a surface and said z-gradient is substantially perpendicular to said surface at said spread spectrum voxel.
- A method as recited in claim 1 wherein said acquiring step further comprises the steps of:tilting nuclei in said spread spectrum voxel;refocusing said tilted nuclei at a rate proportional to said z-gradient to generate encoded spin echo signals; andreceiving said encoded spin echo signals.
- A method as recited in claim 8 further comprising the step of converting said encoded spin echo signals into an image.
- A method as recited in claim 8 wherein said receiving step includes averaging said encoded spin echo signals.
- A method for acquiring data from a single voxel in a plurality of voxels of a tissue sample positioned in a magnetic field which comprises the steps of:impressing at least one intervoxel gradient on said tissue to encode said voxel; andimposing an intravoxel gradient on said tissue to spread the spectrum of all said encoded voxels over a same range of Larmor frequencies, said intravoxel gradient being greater than said intervoxel gradient.
- A method as recited in claim 11 wherein said intravoxel gradient is a z-gradient, and said intervoxel gradient includes an x-gradient and a y-gradient..
- A method as recited in claim 12 wherein said z-gradient is inherent and maintains a value greater than zero during said imposing step.
- A method as recited in claim 12 wherein said z-gradient is substantially constant during said imposing step and has a value greater than 0.2 gauss per millimeter.
- A method as recited in claim 11 wherein said contiguous voxels form a contour having a surface and said z-gradient is substantially perpendicular to said surface at said encoded voxel.
- A method as recited in claim 11 further comprising the steps of:tilting nuclei in said encoded voxel;refocusing said tilted nuclei at a rate proportional to said z-gradient to generate encoded spin echo signals; andreceiving said encoded spin echo signals.
- A method as recited in claim 16 wherein said receiving step includes averaging said encoded spin echoes and said method further comprises the step of converting said averaged spin echo signals into an image.
- A method for magnetic resonance processing which comprises the steps of:placing a tissue sample in a magnetic field;using a first bandwidth on a contour of said tissue to distinguish voxels therein; andusing a second bandwidth on said tissue contour for spreading the spectrum of voxels therein over a same range of Larmor frequencies for reception of spin echo signals therefrom, said second bandwidth being greater than said first bandwidth, and said second bandwidth being extensive to suppress exogenous noise.
- A method as recited in claim 18 further comprising the steps of:tilting nuclei in said voxels;refocusing said tilted nuclei at a rate proportional to said second bandwidth to generate encoded spin echo signals from said voxels;receiving said encoded spin echo signals; andconverting said encoded spin echo signals into useable data.
- A method as recited in claim 19 wherein said steps of using, tilting, refocusing, receiving and converting are repeated for a plurality of said voxels.
- A method as recited in claim 20 wherein said receiving step includes averaging said encoded spin echo signals, and said converting step includes using said data to create an image of said tissue.
- A method as recited in claim 19 wherein said first bandwidth results from an x-gradient and a y-gradient, and said second bandwidth results from a z-gradient.
- A method as recited in claim 22 wherein said z-gradient is substantially constant and has a value greater than 0.2 gauss per millimeter during said receiving step.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
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EP03075290A EP1308743A1 (en) | 1997-07-24 | 1998-07-16 | Spread spectrum MRI |
Applications Claiming Priority (2)
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US08/933,612 US5999838A (en) | 1997-07-24 | 1997-07-24 | Spread spectrum MRI |
US933612 | 1997-07-24 |
Related Child Applications (1)
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EP03075290A Division EP1308743A1 (en) | 1997-07-24 | 1998-07-16 | Spread spectrum MRI |
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EP0893701A1 true EP0893701A1 (en) | 1999-01-27 |
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EP98305679A Ceased EP0893701A1 (en) | 1997-07-24 | 1998-07-16 | Spread spectrum MRI |
EP03075290A Withdrawn EP1308743A1 (en) | 1997-07-24 | 1998-07-16 | Spread spectrum MRI |
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EP03075290A Withdrawn EP1308743A1 (en) | 1997-07-24 | 1998-07-16 | Spread spectrum MRI |
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US (1) | US5999838A (en) |
EP (2) | EP0893701A1 (en) |
JP (1) | JPH11113882A (en) |
AR (1) | AR010934A1 (en) |
AU (1) | AU735232B2 (en) |
BR (1) | BR9815735A (en) |
CA (1) | CA2240683A1 (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP1340093A1 (en) * | 2000-11-06 | 2003-09-03 | Topspin Medical (Israel) Limited | Magnetic resonance imaging device |
Families Citing this family (6)
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US7254451B2 (en) * | 2003-11-20 | 2007-08-07 | Medtronic, Inc. | Implantable lead including sensor |
US20060155186A1 (en) * | 2005-01-12 | 2006-07-13 | James Timothy W | Bone health assessment using spatial-frequency analysis |
US20060241377A1 (en) * | 2005-03-08 | 2006-10-26 | James Timothy W | System and method for bone strength assessment |
RU2423718C2 (en) * | 2005-11-27 | 2011-07-10 | Эквитас Медикал Лимитед | Structure assessment using spatial-frequency analysis |
JP5069693B2 (en) * | 2005-12-23 | 2012-11-07 | コーニンクレッカ フィリップス エレクトロニクス エヌ ヴィ | Method and configuration for wireless communication of signals in an MR system |
EP2207571A4 (en) * | 2007-05-02 | 2011-11-23 | Derek D Feng | Quantum theory-based continuous precision nmr/mri: method and apparatus |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0399789A2 (en) * | 1989-05-22 | 1990-11-28 | The Regents Of The University Of California | Fringe field MRI |
US5304930A (en) * | 1993-02-01 | 1994-04-19 | Panacea Medical Laboratories | Remotely positioned MRI system |
US5517118A (en) * | 1994-04-25 | 1996-05-14 | Panacea Medical Laboratories | Subslicing for remotely positioned MRI |
Family Cites Families (9)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4296378A (en) * | 1979-04-05 | 1981-10-20 | Southwest Research Institute | Apparatus providing enhanced detection of specimens in inhomogeneous fields |
US4471306A (en) * | 1982-02-03 | 1984-09-11 | General Electric Company | Method of NMR imaging which overcomes T2 * effects in an inhomogeneous static magnetic field |
IL68344A (en) * | 1983-04-10 | 1988-10-31 | Yeda Res & Dev | Method to eliminate the effects of magnetic field inhomogeneities in nmr imaging |
US4542343A (en) * | 1983-06-17 | 1985-09-17 | Chevron Research Company | Computer-controlled, portable pulsed NMR instrument and method of use wherein depolarization is ultrashortened using pseudo randomly spaced and oriented RF depolarization pulses |
US4689567A (en) * | 1984-06-01 | 1987-08-25 | Advanced Nmr Systems, Inc. | NMR Fourier imaging from multiple echoes |
US5099208A (en) * | 1989-10-05 | 1992-03-24 | Vanderbilt University | Method for magnetic resonance imaging and related apparatus |
US5150052A (en) * | 1991-02-15 | 1992-09-22 | Meyerand Mary E | Apparatus and method for magnetic resonance spectral imaging |
US5493225A (en) * | 1993-12-14 | 1996-02-20 | Panacea Medical Laboratories | Method for maintaining encoded coherence for remotely positioned MRI device |
JPH10505780A (en) * | 1995-07-20 | 1998-06-09 | フィリップス エレクトロニクス エヌ ベー | Method and apparatus for imaging a curved part of a body using magnetic resonance |
-
1997
- 1997-07-24 US US08/933,612 patent/US5999838A/en not_active Expired - Fee Related
-
1998
- 1998-07-09 CA CA002240683A patent/CA2240683A1/en not_active Abandoned
- 1998-07-16 EP EP98305679A patent/EP0893701A1/en not_active Ceased
- 1998-07-16 EP EP03075290A patent/EP1308743A1/en not_active Withdrawn
- 1998-07-22 BR BR9815735-3A patent/BR9815735A/en not_active Application Discontinuation
- 1998-07-22 AU AU77441/98A patent/AU735232B2/en not_active Ceased
- 1998-07-23 JP JP10207791A patent/JPH11113882A/en active Pending
- 1998-07-24 AR ARP980103639A patent/AR010934A1/en unknown
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0399789A2 (en) * | 1989-05-22 | 1990-11-28 | The Regents Of The University Of California | Fringe field MRI |
US5304930A (en) * | 1993-02-01 | 1994-04-19 | Panacea Medical Laboratories | Remotely positioned MRI system |
US5517118A (en) * | 1994-04-25 | 1996-05-14 | Panacea Medical Laboratories | Subslicing for remotely positioned MRI |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP1340093A1 (en) * | 2000-11-06 | 2003-09-03 | Topspin Medical (Israel) Limited | Magnetic resonance imaging device |
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AU735232B2 (en) | 2001-07-05 |
AU7744198A (en) | 1999-02-11 |
CA2240683A1 (en) | 1999-01-24 |
JPH11113882A (en) | 1999-04-27 |
AR010934A1 (en) | 2000-07-12 |
EP1308743A1 (en) | 2003-05-07 |
BR9815735A (en) | 2001-01-09 |
US5999838A (en) | 1999-12-07 |
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